Metal foils have been developed for a variety of purposes, and are widely used in homes and industries. Aluminum (Al) foils have been widely used for domestic use or for cooking, while stainless steel foils have been commonly used for architectural interior materials or exterior materials. Electrolytic copper foils have been widely used as a circuit of a printed circuit board (PCB). Recently, electrolytic copper foils are being widely used for small devices, such as laptop computers, personal digital assistants (PDA), electronic books, mobile phones, or the like. Metal foils used for special purposes have been manufactured. Iron (Fe)-nickel (Ni) alloy metal foils, among such metal foils have a relatively low coefficient of thermal expansion (CTE), thereby being used as encapsulants for organic light emitting diodes (OLED), an electronic device substrates, or the like. In addition, there is high demand for Fe—Ni alloy metal foils as cathode current collectors and lead frames of secondary batteries.
As a method of manufacturing such Fe—Ni alloy metal foils, a rolling method and an electroforming method have been widely known.
Among them, in the case of a rolling method, after Fe and Ni are cast to be ingots, Fe and Ni are manufactured to be metal foils in such a manner that rolling and annealing is repeated. Since Fe—Ni alloy metal foils manufactured using such a rolling method have a relatively high elongation rate and a smooth surface, cracks may not occur. However, due to mechanical limitations when being manufactured, Fe—Ni alloy metal foils having a width of 1 m or greater are difficult to manufacture, and manufacturing costs thereof are significantly high. In addition, even in a case in which metal foils are manufactured using a rolling method, despite a disadvantage in terms of manufacturing costs, an average grain size of microstructure thereof is coarse, so that mechanical strength properties may be relatively low.
In the meantime, in the case of an electroforming method, metal foils are manufactured in such a manner that an electric current is applied thereto by supplying an electrolyte through an injecting nozzle disposed in a gap between a rotating cylindrical cathode drum disposed in an interior of an electrolytic cell, and a pair of anodes, facing each other and having an arc shape, thereby electrodepositing Fe—Ni alloy metal foils on a surface of the cathode drum to wind the cathode drum. Fe—Ni alloy metal foils manufactured using an electroforming method have a small average grain size, so that mechanical strength properties thereof are relatively high. In addition, since Fe—Ni alloy metal foils may be manufactured using relatively low manufacturing expenses, manufacturing costs thereof are relatively low.
However, in order to use Fe—Ni alloy metal foils manufactured using an electroforming method as encapsulants of organic light emitting devices (OLED), electronic device substrates, or the like, heat treatment at a specific temperature is inevitable. However, in a case in which Fe—Ni alloy metal foils are used in a newly manufactured state thereof, significant thermal deformation occurs when Fe—Ni alloy metal foils are cooled at room temperature after heat treatment at a specific temperature. Such thermal deformation causes contraction greater than that found in a state thereof immediately after Fe—Ni alloy metal foils are manufactured, thereby making a length thereof different from a desired length.